One form of power transmission that has found general acceptance for transferring drive from one member to an adjoining concentric member is the so called recirculating ball-nut screw device. Such devices provide low-friction, low wear and efficient operation under varying temperature and humidity conditions, and were first used on aircraft for moving the wing flaps from a stowed position to an extended position and vice versa. In general, devices of this type consist of recirculating balls guided within a helical ball duct defined between a screw and nut with the balls being continuously recirculated throught the helical duct and through ball return means provided on the nut. Apart from the aviation field, the recirculating ball-nut screw device has many other uses as a linear motion device replacing the threaded nut-and-screw jackshaft-type mechanism. In these uses, the non-rotating nut is typically attached to the linearly movable part of the mechanism while the screw rotates to drive the nut. There are some cases in the machine design field where the use of the recirculating ball-nut screw device could be the ideal choice but cannot be used because of the bulk and disposition of the recirculating ball-return duct interfering with another part of the mechanism.
One form of a ball-nut screw device that eliminates the need for recirculating the balls can be seen in U.S. Pat. No. 4,199,999 issued on Apr. 29, 1980 in the name of Joseph R. Metz. This patent discloses a ball-type mechanical transmission which includes a driver shaft having a rolled-in helical thread engaged by several balls carried in through-openings formed in a sleeve surrounding the shaft. A cylindrical jacket encircles the sleeve so as to provide a back-up bearing surface for the balls and also constitutes a slide having intermediary bearing means within an outer housing. The housing further provides bearings for the shaft and for a connector part constituting the output of the transmission.
Although the above mentioned Metz patent provides a ball-nut transmission without requiring the balls to be recirculated, the use of a separate sleeve for holding the balls and a separate cylindrical jacket sleeve for maintaining the balls in the sleeve openings increases the complexity and cost of the transmission. In addition, by positioning the balls within a cylindrical opening in the sleeve as taught by the Metz patent, a point contact exists between each ball and the surface of the jacket and a circular line contact exists between the ball and the inner edge of the cylindrical opening in the sleeve. The problem with the point contact and line contact between the parts is that high stress concentrations are created in those areas resulting in increased wear in the assembly.